U.S. patent number 4,453,133 [Application Number 06/365,313] was granted by the patent office on 1984-06-05 for active predistorter for linearity compensation.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to George W. Travis.
United States Patent |
4,453,133 |
Travis |
June 5, 1984 |
Active predistorter for linearity compensation
Abstract
A wideband amplifier utilizes predistortion components developed
by an amplifying device (14) similar to the power amplifying device
(15) to provide effective linearization for substantially reducing
all of the intermodulation distortion products. Since the
compensation is provided at the input to the power amplifier, the
need for an output signal combiner is obviated so that the total
power is available at the output. The efficiency is also enhanced
since the predistortion amplifier (14) which operates from the
input signal ony need be a voltage amplifier. An optional feedback
circuit is disclosed which may be employed to reduce nonlinearities
further if necessary.
Inventors: |
Travis; George W. (Colts Neck,
NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
23438359 |
Appl.
No.: |
06/365,313 |
Filed: |
April 5, 1982 |
Current U.S.
Class: |
330/149; 327/100;
330/151 |
Current CPC
Class: |
H03F
1/32 (20130101); H03F 1/3252 (20130101); H03F
1/3229 (20130101) |
Current International
Class: |
H03F
1/32 (20060101); H03F 001/26 () |
Field of
Search: |
;330/75,84,149,151
;328/163 ;332/18,37R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mullins; James B.
Assistant Examiner: Mottola; Steven J.
Attorney, Agent or Firm: Moran; John Francis
Claims
What is claimed is:
1. An amplifier circuit for an electromagnetic wave signal
comprising a power amplifier and an auxiliary amplifier wherein the
distortion introduced by the power amplifier is compensated by
using the auxiliary amplifier characterized in that
the auxiliary amplifier is connected to receive a portion of the
electromagnetic wave signal and is a voltage amplifier having
similar input-output, delay, and phase characteristics to the power
amplifier, means for combining the output of the auxiliary
amplifier and a portion of the electromagnetic wave signal in phase
opposition relationship to produce an output of distortion
components substantially free of the presence of the
electromagnetic wave signal, and means for adjusting the electrical
path length traversed by the electromagnetic wave signal before
being applied to the input of the power amplifier, and means for
combining the distortion components and the electromagnetic wave
signal to provide predistortion compensation for improving the
linearity of the amplifier circuit.
2. An amplifier in accordance with claim 1, wherein said power
amplifier and said auxiliary amplifier comprises the same kind of
amplifying devices.
3. An amplifier in accordance with claim 2 further comprising
attenuating means connected to receive the distortion components
for adjusting their amplitude to effect linearization of the
operating characteristic of said power amplifier.
4. An amplifier in accordance with claim 3 wherein said power
amplifier and said auxiliary amplifiers both comprise field effect
transistor amplifying devices.
5. An amplifier in accordance with claim 3 wherein said power
amplifier and said auxiliary amplifier both comprise traveling wave
tube amplifying devices.
6. An amplifier in accordance with claim 2 further comprising a
second signal combiner connected to receive a portion of the
electromagnetic wave signal and a portion of the output signal of
said main amplifier for producing a feedback signal for the input
of said power amplifier for providing further linearization of its
operational characteristic.
7. An amplifier in accordance with claim 6 wherein connected in
circuit with the second signal combiner are attenuating means and
phase shifting means for providing a portion of the output signal
from the power amplifier indicative of its nonlinearity
appropriately adjusted in amplitude and phase to enhance further
the linearization of the amplifier.
Description
BACKGROUND OF THE INVENTION
This invention relates to linearity compensation used to reduce
distortion in amplifiers. More particularly, the invention relates
to feedforward techniques in broadband amplifiers employing similar
types of amplifying devices.
Various distortion compensation techniques have been devised for
improving the performance of amplifiers. Among them are feedforward
and feedback. Conventional feedforward arrangements require
duplication of power amplifiers, each in a signal path that are
combined at the output. The amplifier duplication wastes power
while the loss incurred by combining signals at the output reduces
the effective output of the amplifier. Feedback, on the other hand,
reduces distortion at the expense of gain often requiring further
stages of amplification to produce the desired output.
In some applications more than others, such as satellite repeaters,
these penalties are serious drawbacks. Of chief concern are compact
circuits with low power consumption and minimal weight for use in
satellites. Another concern is that the circuits in satellites are
typically operated at high levels to obtain high output levels for
the total power consumed which unfortunately tends to exacerbate
the effect of nonlinearities.
Due to the ever increasing demand in the amount of information to
be transmitted, the need for linear amplifiers with wide bandpass
characteristics is highly desirable. Currently, transponders
operating in designated bands in a satellite repeater are required
to prevent intermodulation distortion of multipath interference
between received signals being amplified and redirected toward
earth stations. Such an approach is costly in terms of the loss of
bandwidth efficiency due to the required spacing between designated
bands and the weight of the several sections in the filter
demultiplexers used in such an approach.
Another linearization technique is known as predistortion
compensation wherein predistortion components are added to the
input signal to provide destructive interference between the
predistortion components and a selected order of the distortion
introduced by the amplifier while in the process of amplifying.
Conventional arrangements, however, typically utilize passive
circuitry (e.g., diode arrangements) in the predistorter which
reduces distortion by partially emulating the distortion
characteristics of the active amplifier. In effect, a large portion
of the input signal is required to provide sufficient signal levels
for operating the predistorter before amplification to generate a
particular order of distortion components. As a result, the gain or
amplification factor is reduced to improve performance for the
selected order of distortion.
Although these drawbacks are particularly acute in satellite
applications, an amplifier which overcomes such problems will offer
attendant advantages that are highly beneficial in a variety of
other applications wherein high linearity and wide bandpass
characteristics are desirable. For example, typical applications
may include microwave radio and single sideband transmission
systems.
SUMMARY OF THE INVENTION
Broadly, the invention takes the form of a predistortion
compensator employing a voltage amplifier for producing distortion
components which are combined in appropriate amplitude and phase
relationship with the input signal of a power amplifier to
linearize its operation.
In some of the additional aspects of the invention, the voltage
amplifier is selected to have similar input-output, delay and phase
characteristics as the power amplifier. By taking the difference
between a portion of the output and the input of the voltage
amplifier, distortion components are produced for adding to the
input signal of the power amplifier to achieve linearity
compensation. Both amplifiers may take the form, for example, of a
traveling wave tube or a field effect transistor.
In some of the further aspects of the invention, an optional
feedback loop is utilized to provide an additional reduction in the
distortion produced by slight variations between the
characteristics of the voltage and power amplifiers. In the loop,
the difference between a portion of the output signal and the input
signal is also added to the input signal of the power amplifier for
correcting its operation. Due to the additional delay of the
feedback loop in this variation, broadbanding will be somewhat
limited.
BRIEF DESCRIPTION OF THE DRAWING
The above-mentioned and other features and objects of the present
invention will become more apparent by reference to the following
detailed description taken in conjunction with the accompanying
drawing wherein:
FIG. 1 is a circuit diagram embodying the inventive feedforward
compensation technique, and;
FIG. 2 is also a circuit diagram embodying the invention wherein an
optional feedback arrangement is employed.
DETAILED DESCRIPTION
FIG. 1 is a diagram of circuit designed to provide predistortion
compensation in accordance with the invention for main power
amplifier 15. The input signal is applied to amplifier 15 via
signal splitter 11, delay 12 and signal combiner 13. One of the
other two components of the input signal from signal splitter 11
drives voltage amplifier 14. The remaining output of signal
splitter 11 is retained by delay 16 before application to signal
combiner 17. The other input of signal combiner 17 is a portion of
the output of amplifier 14 provided by coupler 20 via adjustable
attenuator 21. Coupler 21 is impedance terminated by load 18.
The sole purpose of amplifier 14 is to generate the same type of
distortion components as produced by the nonlinearity of amplifier
15. Although amplifier 14 is a preamplifier or voltage amplifier,
it is selected to have similar transfer characteristics to
amplifier 15. Accordingly, the input-output, delay and phase
characteristics of the two amplifiers should be similar. Similarly
of characteristics of amplifiers is most typically obtained when
the same fabricating technique is used in making both devices.
Signal combiner 17 takes the difference between a delayed version
of the input signal and a portion of the output signal of amplifier
14 which is adjusted to have the same amplitude by using attenuator
21. The output of signal combiner 17 thus produces only the
distortion components attributed to the nonlinearity distortion
introduced by amplifier 14.
The distortion component output from signal combiner 17 is adjusted
in amplitude by attenuator 22. The relative phase between the
distortion component and the input signal to signal splitter 11 is
adjusted by delay 12. The predistorted input signal of amplifier 15
is produced by the output of signal combiner 13. When the
distortion components produced by amplifier 14 are adjusted to have
the appropriate phase and amplitude, predistortion compensation is
achieved for amplifier 15. Accordingly, as amplifier 15 is
increasing its output in response to the amplitude of its primary
input signal from delay 12, the presence of predistortion
components applied to its input serves to cancel out the distortion
components that are normally produced by the nonlinearity of
amplifier 15.
As a result, due to this circuit arrangement, the output signal of
amplifier 15 is a more faithful reproduction of the input signal
applied to signal splitter 11 which is due to correcting the
nonlinearity present in amplifier 15. Amplifier 14, which primarily
serves as a voltage amplifier, is thus able to correct or linearize
the performance of a power output amplifier such as amplifier 15.
Accordingly, the circuit dissipates a minimum of power and
virtually the full output of amplifier 15 is available for
utilization.
It should be noted that it is important for broadbanding that the
effective electrical path length from signal splitter 11 to signal
combiner 17 via amplifier 14, coupler 20 and attenuator 21 be equal
to the electrical path length between signal splitter 11 to
combiner 17 via delay 16. Also, the electrical path length from
splitter 11 to combiner 17 when added to the electrical path length
from combiner 17 to combiner 13 via attenuator 22 should be equal
to the electrical path length from signal splitter 11 to combiner
13 via the path of delay 12.
FIG. 2 employs feedback compensation in addition to predistortion
compensation. Like reference numerals in the drawing figures are
utilized to identify like electrical components. In FIG. 1, the
assumption was made that the preamplifier and power amplifier have
similar transfer characteristics which is proper when similarly
constructed traveling wave tubes, klystrons, power field effect
transistors and the like are utilized. However, the arrangement of
FIG. 2 dynamically removes any residual distortion produced by
slight differences between the operational characteristics of the
low-power voltage amplifier and the high-power amplifier.
One of the differences between FIGS. 1 and 2 resides in that signal
splitter 11 provides an additional output. Signal comparator 26
receives this signal via adjustable delay 27 and a portion of the
output signal from power amplifier 15. The latter input is obtained
from the output of the power amplifier via signal splitter or
directional coupler 28 and attenuator 31. The two input signals to
comparator 26 are appropriately adjusted to be in amplitude
coincidence and 180 degrees out of phase so that distortion is the
only output of comparator 26. The distortion is coupled to the
input of amplifier 15 through adjustable attenuator 32 and an
additional input of signal combiner 13. Since this feedback loop
operates from a residual effect due to incomplete cancellation
which results in low level feedback signals, the feedback loop may
be readily implemented. Accordingly, the components of the feedback
loop may not be required to operate over a large dynamic range and
the available bandwidth will be sufficient for the frequency
content of the signal involved.
Although the invention provides advantages highly desirable for
satellite applications, it is again stressed that the benefits
accrued will enable those skilled in the art to utilize the
invention to advantage in various other applications. Also,
modifications of the illustrative embodiments will become apparent
to those skilled in the art for practicing the inventive
principles. For example, in some applications the operational
characteristics of dissimilar devices, such as a klystron and
traveling wave tube, may be sufficiently close to yield adequate
linearization. Although the embodiments of the invention are
illustrated using adjustable components, proper amplitude and phase
relationships also may be obtained by judicious selection of
circuit components and circuit layout techniques to reduce
adjustments and to achieve economy. In all cases, numerous and
varied other arrangements may become apparent to those working in
the invention.
* * * * *